Internet access over CATV systems is typically accomplished using a cable modem. As an industry standard, the Data Over Cable Interface Specification (DOCIS) has emerged to provide interoperability between cable modems from diverse manufacturers. Internet access by cable modem provides generally higher download data rates than dial up modem access over POTS (Plain Old Telephone Service), ISDN (Integrated Services Digital Network) or DSL (Digital Subscriber Line) services. A typical cable modem is frequency agile and utilizes the unused portions of the CATV frequency spectrum to transmit signals in the upstream and downstream directions. While cable modems are often stand alone boxes, some CATV settop boxes are provided with a built-in cable modem.
In the downstream direction, (from the headend to the subscriber) cable modems make use of unused frequencies in the available frequency spectrum above 50 MHz. Most of the spectrum above 50 MHz is used for the delivery of video programs. A typical CATV system may have spectrum usable for data above the upper limit usable for video (say above 500 MHz for many systems or above 750–1000 MHz for newer systems). Also, a CATV system has an unused mid-band frequency region between 70 MHz and 130 MHz that does not contain video channels. The downstream data capacity of a CATV system is thus confined to the 70–130 MHz band and the band above the highest video channel. While an analog video signal may carry some digital data (such as data in the vertical blanking interval), the data carrying capacity of a standard analog format TV signal is severely limited. Although any video channel could be pre-empted and used to transmit downstream data, CATV operators are reluctant to convert a video channel to an all data channel because it reduces the video capacity of the CATV system.
The return path (the upstream direction from the subscriber to the headend) of a CATV system is the frequency band below 50 MHz. Typically, only a portion of the upstream return spectrum, from 5 to 30 MHz, is used, leaving a guard band of 20 MHz between the highest upstream frequency (30 MHz) and lowest downstream frequency (50 MHz). Unusable frequencies below 5 MHz are excluded. The non-video frequencies (in this case, 5–30 MHz, 70–130 Mhz and those above the highest video signal) are referred to herein as the out-of-band (OOB) frequencies. The video channels are organized as 6 MHz frequency bands, each carrying one NTSC analog video signal.
A cable modem may thus operate on any available pair of frequencies, one downstream out-of-band frequency and one upstream out-of-band frequency. The subscriber (viewer) may use the resulting two-way communication over the CATV system solely for Internet access via cable modem. However, TV video content and Internet content may be linked or related to each other. For example, a video sports event may access the Internet for sports statistics related to the video sports event. A TV advertisement may reference a related Web site, or accept direct orders for merchandise over the Internet. However, if the subscriber desires to access the Internet via a cable modem and also receive cable TV video at the same time, then two tuners are required: one for tuning a video signal an another for tuning the cable modem signal.
CATV systems typically transmit digital video in MPEG-2 format, which makes better use of the available CATV spectrum as compared to analog signals. A 6 MHz video channel, which normally carries one standard definition analog channel, may carry at least 4 multiplexed digital video programs or at least 1 high definition digital video program. In addition to expanded video capacity, digital multiplexed MPEG-2 video standards include capacity for carrying Internet Protocol data (IP over MPEG). The private data portions of multiplexed MPEG-2 video may carry Internet Protocol (IP) over the same channel with simultaneous multiplexed MPEG-2 digital TV signals. In such manner, the in-band frequencies (the video channels) carry downstream Internet data along with cable TV digital video.
However, the viewer may be receiving IP over MPEG on one 6 MHz video channel and desire to watch a digital TV program that is multiplexed on another 6 MHz video channel. Therefore, two tuners and two digital MPEG decoders are needed if the subscriber is to be able to simultaneously receive both IP over MPEG Internet data on any digital video channel and view a digital video program from another MPEG encoded 6 MHz video channel. It would be desirable to utilize the downstream data carrying capacity of the in-band CATV spectrum to simultaneously receive both IP over MPEG Internet data and MPEG encoded digital video using a single tuner/digital MPEG decoder in the CATV settop box.